Fluoropolymers are well known for their ability to withstand strenuous environments, because they are substantially completely nonreactive with most chemical reagents including most acids, bases, oxidizing agents, and reducing agents. For this reason, fluoropolymers, particularly fluoropolymers such as polytetrafluoroethylene and copolymers of tetrafluoroethylene and hexafluoropropylene or perfluoroalkyl vinyl ethers, are used extensively in applications requiring chemical inertness. Besides inertness, fluoropolymers are well known for their heat stability, with maximum service temperatures as high as 500.degree. F. (260.degree. C.).
Despite the many desirable properties of fluoropolymers, they have certain disadvantages. They are expensive and, in certain cases, have deficiencies as structural plastics. For these and other reasons, it is desirable to have technology to efficiently coat substrates with fluoropolymers so that the positive features of the fluoropolymers and the substrates can be used while minimizing their inherent disadvantages. However, untreated fluoropolymers cannot be efficiently and effectively bonded to other materials using conventional adhesives. Sometimes fluoropolymers can be applied and hot melted to other materials, but the coated material must be able to withstand the heat necessary to melt or soften the fluoropolymer. Furthermore, fluoropolymers applied as hot melts often fail upon temperature cycling and separate from the substrate as a result of differences in thermal expansion coefficients between the fluoropolymer and the substrate on which it is coated. Fabrication of fluoropolymer-coated large objects or thick fluoropolymer coatings on substrates presents additional difficulties because large objects are difficult to heat evenly across their entire mass and because some layers of fluoropolymer degrade under the heat used for applying additional coatings. Historically, several approaches have been used to bond fluoropolymers to substrates. These methods include sodium etching, plasma treatments, photochemical treatments, and mechanical abrasion. Such methods are discussed in Polymer Interface and Adhesion, by Souheng Wu, Marcel Dekker, Inc., N.Y., 1982, pages 280, 283, 298-299, 304, 322-323. A need exists for a simpler, more efficient method to bond fluoropolymers to other materials. The present invention provides such a method.